JPH032438B2 - - Google Patents

Description

Detailed Description of the Invention <Field of Industrial Application> The present invention relates to a loop-type fast breeder reactor, and more specifically, to reduction of thermal shock in overflow piping of a reactor vessel and improvement of coolant in the reactor vessel. This invention relates to a loop-type fast breeder reactor that is capable of quickly eliminating the thermal stratification phenomenon.

<Prior art> As shown in FIG. 4, the primary cooling system of a loop type fast breeder reactor uses a primary coolant (for example, sodium ) flows through the reactor core 3, flows out from the primary coolant outlet pipe 4 at the top of the reactor vessel 1, passes through a heat exchanger installed outside the reactor and a primary main circulation pump (none of which are shown), and then returns again. The coolant is configured to circulate from the coolant inlet pipe 2 into the reactor vessel 1 .

In addition to this primary cooling system, there is a system that absorbs or replenishes changes in the sodium capacity of the cooling system due to operating conditions of the primary main circulation pump or temperature changes in the cooling system to keep the liquid level L in the reactor vessel 1 constant at all times. An overflow system is provided. This overflow system guides excess sodium overflowing from an overflow pipe 5 disposed above the primary coolant outlet pipe 4 to the outside of the reactor vessel 1 through an overflow return pipe 6 to an overflow tank 7.
At the same time, the sodium in the overflow tank 7 is pumped up into the reactor vessel 1 by an electromagnetic pump 8.

In such fast breeder reactors, the temperature of the cooling system during reactor power operation is extremely high, reaching temperatures of over 500°C in the primary cooling system. In this state, if a reactor scram (such as an external power loss accident) occurs that causes the primary main circulation pump to stop, the sodium level in the reactor vessel 1 will decrease due to the loss of the liquid level pushing force by the primary main circulation pump and a sudden Rapidly decreases due to heat shrinkage. Therefore, the flow of sodium overflowing from the overflow pipe 5 is interrupted, and a rapid temperature drop phenomenon occurs in the overflow return pipe 6. On the other hand, the sodium in the upper layer of the reactor vessel 1 becomes hot and stagnates (thermal stratification phenomenon) due to a decrease in the stirring power of the primary main circulation pump. In such a situation, when the overflow system electromagnetic pump 8 pumps up sodium to restore the sodium level in the reactor vessel 1 and overflow resumes, high-temperature sodium flows into the overflow return pipe 6, which has experienced a rapid temperature drop. flows, resulting in thermal shock.

<Problems to be Solved by the Invention> In order to prevent damage to piping, etc. due to such thermal shock, in the past, when a reactor scram occurred that required the primary main circulation pump to stop, the overflow system electromagnetic pump 8, the recovery of the sodium level in the reactor vessel 1 was stopped before the overflow level, and then the temperature of the cooling system was lowered and after confirming that no thermal shock occurred in any part, pumping of the overflow system was started. The overflow from the overflow pipe 5 was restarted.

For this reason, the reactor cannot be restarted after it has been shut down until the temperature of the cooling system has dropped, so it takes more than 10 hours to restart the reactor after it has been shut down.

Therefore, this invention prevents the thermal shock that occurs in the overflow return piping to prevent damage to the piping, and also quickly eliminates the thermal stratification phenomenon that occurs inside the reactor vessel, and restarts the reactor after shutdown. This was done with the aim of providing a loop-type fast breeder reactor that can operate more efficiently in a shorter period of time.

<Means for Solving the Problems> That is, the loop fast breeder reactor according to the present invention includes a primary coolant inlet pipe at the bottom of the reactor vessel, a primary coolant outlet pipe at the top of the reactor vessel, and A conventional loop-type high speed system as shown in Fig. 4 is equipped with an overflow pipe for the coolant inside the reactor vessel disposed above the coolant outlet pipe and an overflow return pipe that guides the overflow coolant to the outside of the reactor vessel. In a breeder reactor, a heat exchanger is disposed in a low temperature sodium layer in the reactor vessel below the upper level of the coolant outlet piping opening, and the lower end of the overflow pipe is connected to the upper inlet of the heat exchanger. This is characterized in that the upper ends of the overflow return pipes are connected to the lower end outlets of the heat exchanger.

<Embodiments> The present invention will be described in detail below with reference to embodiments shown in the drawings.

Figure 1 shows the primary coolant outlet pipe 4 in Figure 4.
4 shows an enlarged view of the vicinity of the overflow pipe 5, and the same members as in FIG. 4 are denoted by the same reference numerals as in FIG. 4, and their explanation will be omitted.

The difference from the conventional fast breeder reactor shown in FIG. This is the point where the lower end of the heat exchanger 10 is connected to the upper end inlet, and the upper end of the overflow return pipe 6 is connected to the lower end outlet of the heat exchanger 10.

The heat exchanger 10 is arranged at an angle so as not to affect the overflow capacity, and is a multi-tubular heat exchanger with fins as shown in FIG. 2 to increase the heat transfer area. It is preferable.

By installing such a heat exchanger 10, the high temperature sodium layer A in the upper sodium layer inside the reactor vessel 1
The high-temperature sodium that has overflowed through the overflow pipe 5 exchanges heat with the sodium in the relatively low-temperature sodium layer B located below the upper end level 4a of the coolant outlet pipe 4, and reaches a temperature close to that of the low-temperature sodium layer B. After the temperature has been lowered to 100%, the overflow is led to the overflow return pipe 6.
From this point of view, the installation position of the heat exchanger 10 is
The upper end inlet of the heat exchanger is located below the upper end level 4a of the coolant outlet pipe 4 opening, and the lower end outlet of the heat exchanger is located at the lower end level 4b of the coolant outlet pipe 4 opening.
It is desirable to have the same height as the

Transient changes in an overflow system provided with the heat exchanger 10 as described above will be explained with reference to the graph of FIG. 3. If a reactor scram (external power loss accident) occurs during reactor power operation with the stoppage of the primary main circulation pump, the sodium liquid level L in the reactor vessel 1 will increase due to the liquid level pushing up force due to the stoppage of the main circulation pump. It rapidly decreases due to the decrease in the temperature and the thermal contraction of sodium due to scram. As a result, the sodium flow in the overflow return pipe 6 is interrupted, and the temperature within this pipe drops rapidly at a rate of drop of 6 to 7°C/min. On the other hand, due to the reduction in the amount of sodium circulating in the reactor vessel due to the stoppage of the main circulation pump, a thermal stratification phenomenon occurs in which high-temperature sodium accumulates in the upper layer.
The temperature of this upper sodium layer is maintained at a high temperature comparable to that during reactor power operation, whereas the reactor outlet sodium temperature rapidly decreases at the same time as the reactor scram. By pumping up sodium from the overflow tank 7 of the overflow system from this state, the sodium liquid level L in the reactor vessel recovers and reaches the re-overflow point P. At this point, when high-temperature sodium in the upper part of the reactor is introduced directly from the overflow pipe 5 to the overflow return pipe 6, a thermal shock due to the temperature difference (approximately 80°C) from point Q to point R is applied to the overflow return pipe 6. will occur.

However, in this invention, the reactor upper layer A that overflowed from the overflow pipe 5
The high temperature sodium flows through the heat exchanger 10 and is introduced into the overflow return pipe 6. At this time, since the sodium temperature in the low-temperature sodium layer B below the upper end level 4a of the coolant outlet pipe 4 is as low as the reactor outlet sodium temperature, the sodium flows through the heat exchanger 10 installed in the low-temperature sodium layer B. The high temperature overflow sodium undergoes heat exchange and is lowered in temperature to near the reactor outlet sodium temperature before being led to the overflow return pipe 6. As a result, the thermal shock that occurs in the overflow return pipe is reduced by the temperature difference from point S to point R (approximately 20
°C).

<Effects of the Invention> As explained above, when a reactor scram occurs during output operation of a loop type fast breeder reactor and the primary main circulation pump is stopped, if the overflow system is operated continuously, Because it was expected that a thermal shock of approximately 80°C would occur in the overflow return piping, it was necessary to restart the overflow approximately 10 hours after the reactor was shut down. However, as in this invention, even if the overflow system is operated continuously and the overflow is restarted by installing an in-reactor vessel heat exchanger between the overflow pipe and the overflow return pipe, Thermal shock to the overflow return pipe can be reduced to about 20°C.

As a result, the reactor restart time traditionally was 12-13
What used to take time can be reduced to less than an hour.
Furthermore, the time required to reach the rated output can be further shortened by 2 to 3 hours since the time required to raise the temperature of the cooling system can be further reduced, contributing to an improvement in the operating rate.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing an embodiment of the present invention;
The figure is a plan view of the heat exchanger part in Figure 1,
The figure is a graph showing transient changes in the overflow system in the present invention, and FIG. 4 is an explanatory diagram showing the overflow system of a conventional loop type fast breeder reactor. DESCRIPTION OF SYMBOLS 1... Reactor vessel, 2... Primary coolant inlet piping, 4... Primary coolant outlet piping, 5... Overflow pipe, 6... Overflow return piping, 10
……Heat exchanger.

Claims (1)

[Scope of Claims] 1. A reactor vessel comprising a primary coolant inlet pipe at the bottom of the reactor vessel, a primary coolant outlet pipe at the top of the reactor vessel, and further disposed above the coolant outlet pipe. In a loop fast breeder reactor equipped with an overflow pipe for internal coolant and an overflow return pipe for guiding overflow coolant out of the reactor vessel, the reactor vessel below the upper end level of the coolant outlet pipe opening. A heat exchanger is disposed in the low-temperature sodium layer in the interior, the lower end of the overflow pipe is connected to the upper end inlet of the heat exchanger, and the upper end of the overflow return pipe is connected to the lower end outlet of the heat exchanger. Loop type fast breeder reactor. 2. The heat exchanger is arranged at an angle, and the upper end inlet of the heat exchanger is below the upper end level of the coolant outlet pipe opening, and the lower end outlet of the heat exchanger is below the upper end level of the coolant outlet pipe opening. The loop fast breeder reactor according to claim 1, wherein each of the loop fast breeder reactors is located at a height comparable to the lower end level. 3. The loop fast breeder reactor according to claim 1 or 2, wherein the heat exchanger is a multitubular heat exchanger with fins.